Some sensors and other components of spacecraft and aircraft must be cooled to cryogenic temperatures of about 77° K or less to function properly. A number of approaches are available to perform this cooling, including thermal contact to liquefied gases and cryogenic refrigerators, usually termed cryocoolers. The use of a liquefied gas is ordinarily limited to short-term missions. Cryocoolers typically function by the expansion of a gas, which absorbs heat from the surroundings. Intermediate temperatures in the cooled component may be reached using a single-stage expansion. To reach colder temperatures required for the operation of some sensors, such as about 40° K or less, a multiple-stage expansion cooler is often preferred. The present invention is concerned with applications requiring continuous cooling to such very low temperatures over extended periods of time.
One type of cryocooler used for such applications is a two-stage Stirling/pulse tube cryocooler. Stirling/pulse tube cryocoolers are described, for example, in U.S. Pat. Nos. 6,167,707 and 6,330,800. Briefly, a Stirling expander piston produces refrigeration in the first, warmer, stage. A pulse tube produces refrigeration in the second, colder, stage. Both stages are driven by a pressure wave generated by a reciprocating compressor connected to the inlet of the Stirling-expander first stage.
In the work leading to the present invention, two problems have been observed to limit the performance of the Stirling/pulse tube cryocooler. First, the efficiency falls with lower temperatures, particularly in the pulse tube second stage wherein the working gas is at the lower temperatures. Pulse tube losses consume about 25-40 percent of the gross refrigeration capacity. Second, the compressor pressure wave-to-piston motion phase angle is about ten degrees larger than required for optimal performance.
The Stirling/pulse tube cryocooler has great potential for use in sensor and other cooling applications requiring cooling to low temperatures, but there is a need to overcome these problems to improve its performance even further. The present invention fulfills this need, and further provides related advantages.